Gerbils More Responsible than Rats for the Black Death

In a now iconic scene, almost as famous for the logistical feat of its staging as for its cinematic impact, the 1979 Werner Herzog film Nosferatu the Vampyre features over 10,000 rats swarming a 19th century German town. As people start dying by the score, the town gives itself up to what is assumed to be plague.

But the black rat, or to use its more pleasing Latin name, Rattus rattus, may have unfairly been given a bad reputation in popular and medical history, according to a recent publication in the Proceedings of the National Academy of Sciences. Rats, scientists now say, are not to blame for the multiple reoccurrences of plague between the 14th and 19th centuries. After using climate data to analyze the conditions under which plague resurfaced during this period, they suggest we should perhaps be pointing our fingers at one of the rat’s rodent relatives, the giant Asian gerbil.

In 1347, the bacterium Yersinia pestis reached the ports of Europe. That year marked the onset of the continent’s second plague pandemic, more commonly known as the Black Death, which is estimated to have killed between one and two thirds of the continent’s population over the next four years. Bacteria were transmitted to humans via fleas, and caused several unpleasant conditions including bubonic, pneumonic and septicemic plagues.

In a now iconic scene, almost as famous for the logistical feat of its staging as for its cinematic impact, the 1979 Werner Herzog film Nosferatu the Vampyre features over 10,000 rats swarming a 19th century German town. As people start dying by the score, the town gives itself up to what is assumed to be plague.

Although the first of these three plagues is the most commonly associated with the Black Death, causing lymph glands to swell into painful “buboes” and conferring a mortality rate of up to 75%, many modern scholars now think that pneumonic plague, with a mortality rate of over 90%, also contributed to the high death rate of the original infection of Europe.

In case the first four years of gruesome deaths weren’t enough, disease continued to decimate the European population through repeated, albeit smaller, outbreaks of plague over the next 500 years.

The principal explanation for how the outbreaks recurred relied on black rats as central characters. Rats, so the story went, acted as “reservoirs” for the disease by housing infected fleas on their bodies; the fleas in turn housed the Y. pestis bacterium, and human outbreaks occurred when these rat populations spilled back into human societies.

But the new study, published by researchers at the University of Oslo, throws this whole explanation into question.

To understand more about the spread of plague to Europe from its origin in Asia, the team wanted to measure the climatic conditions that preceded outbreaks. They assembled a collection of ancient tree trunk sections from locations all over Europe and Asia. (If you’ve ever estimated a tree’s age by counting the rings in its trunk, you might see where the researchers were going with this.) They then used tree rings — a measure for annual tree growth — to give them a picture of the climatic conditions during the period that plague was infecting and re-infecting Europe.

When they compared their European climate data to the location and frequency of outbreaks across the continent, they hit upon a problem with the traditional explanation. The researchers knew that outbreaks of disease in humans triggered by rat populations tend to occur in periods with warm and fairly dry weather. But the data they had collected showed that outbreaks were sometimes occurring in conditions that didn’t match this description at all. In fact, there was no significant relationship between the weather and the appearance of plague in Europe.

But what if, the team asked, the outbreaks of plague were being affected by the climate in a different region, one outside Europe altogether? Sure enough, when they analyzed the location and frequency of outbreaks using the Asian climate data, instead of the European climate data, they found a pattern. In their paper, they show that outbreaks of plague in Europe were far more likely to occur following wet springs and warm summers in Asia — with a 15-year delay. In other words, 15 years after this specific set of climactic circumstances occurred in Asia, plague would once again crop up in Europe.

This result, the researchers say, may exonerate black rats from shouldering the entirety of the blame. While it’s quite likely that rats helped to spread the plague between towns and ports, it is far less likely that they were responsible for re-introducing the disease across the five centuries following the 1347 pandemic.

Instead, the data suggest that an Asian rodent, most likely the giant Asian gerbil, Rhombomys opimus, was responsible for triggering multiple transmissions of plague from Asia to Europe. The combination of wet springs and warm summers, according to the paper, can have a strong influence on the prevalence of plague among gerbils. In the 500 years the scientists studied, these conditions could have caused a dramatic spread in the disease across wild populations that later spilled over to domestic animals and humans. The bacterium most likely then slowly made its way to Europe in the following few years via trade routes such as the cross-continent Silk Road, and to multiple ports via trading ships. The researchers identify at least 11 European re-introductions of plague that they believe progressed in this way.

The team says that it next plans to test its theory by studying the DNA of bacteria taken from plague victims of the second pandemic. If the disease were indeed re-introduced in multiple waves, one would expect to see more genetic variation in ancient Y. pestis than if it had simply spread slowly around Europe for the whole period.

But even so, the black rat may have to wait some time for its name to be cleared of harboring the plague that wiped out an estimated 50 million people in the 14th century. Invasive pests in many parts of the world, rats carry a host of other pathogens in addition to Y. pestis, and transmit serious diseases including pneumonia and bacterial meningitis.